Conventional liquid crystal displays for television sets operate at frame frequencies from 50 Hz through 60 Hz. Demands are however growing for those operating at double the frequency, i.e. 100 Hz to 120 Hz, to produce smooth motion pictures. Since liquid crystal is charged line by line according to an active matrix scheme, doubling the frame frequency means simply halving the charging time. A liquid crystal element is equivalent to a capacitor and is only insufficiently charged in half the charging time. The element does not reach the potential required for a display, which will result in an inaccurate grayscale display and poor display quality.
High definition displays needs increased numbers of scan lines representing each frame, reducing charging time per line and causing similar insufficient charging. This degrades display quality. For example, the number of lines of vertical scan lines of conventional television ranged from about 400 to 600. Meanwhile, coming high vision television has double that number at 1080 lines.
These issues are addressed by, for example, the conventional methods disclosed in Japanese published patent applications 2-168229 (Tokukaihei 2-168229/1990; published on Jun. 28, 1990) and 11-38379 (Tokukaihei 11-38379/1999; published on Feb. 12, 1999).
According to Tokukaihei 2-168229, the liquid crystal is driven (scanned) a line at a time; at the same timing, the liquid crystal is preliminarily scanned along at least another line. The preliminary scanning applies a voltage of a predicted value to pixels prior to actual scanning, and in practice adds to the scan period for that line. Thus, this approach prevents image quality degradation due to insufficient ON current.
According to Tokukaihei 11-38379, each scan signal line is fed with a scan signal during a secondary scan period. A data signal during that secondary scan period is diverted for the precharging of a cell. This approach allows a simple structure to cut down on the charging time of a cell in a primary scan period.
These techniques can solve insufficient charging.
However, the conventional liquid crystal display still entails insufficient charging when it is expected to produce a display at such high frame frequencies or a high-definition display with such largely increased numbers of horizontal lines as to halve the charging time.